The invention relates to an extruder, especially to a multi-worm extruder, comprising an extruder housing, one or more mixing worms with associated shaft and at least one hydrostatic bearing for absorbing the axial load of the conveying and mixing worm, while the slit between a bearing disc provided on the worm shaft and a counter bearing ring can be filled with oil via a pipe connected to a pump and radial sealings are provided, preventing the oil from escaping between the gliding faces of the bearing.
Such an extruder with a hydrostatic bearing is known per se. If this known extruder is axially loaded, an oil pressure is built up in the bearing slit, causing the bearing to be hydrostatically supported.
For obtaining a proper bearing, the slit between the bearing disc on the worm shaft and the counter bearing ring, is namely connected with an oil pump with a constant output. This pump pumps the oil towards the slit and as soon as the slit is filled with oil, the worm shaft will be slightly pressed forwards. At that moment the extruder can be switched in, since the sliding faces of the bearing disc and of the counter bearing ring do not longer contact. When the axial power of the worm increases, the slit becomes narrower whereby the oil velocity in the slit increases as well and the pressure becomes greater. A new balance is established then and the slit gets a new width.
Such a bearing shows many drawbacks.
When the oil pressure fails for instance, the oil pressure is discontinued owing to which the sliding faces of the bearing discs on the shaft of the mixing and conveying worm and of the counter bearing ring are pressed on each other rather soon with a great vigor. Consequently the two faces will "freeze", while damaging the counter bearing ring.
Another disadvantage is that when the oil pressure is suddenly discontinued, the synthetic material in the extruder is still at a high temperature, but is no longer mixed and conveyed. This results into decomposition of the synthetic material. This is attended with the formation of solid deposits in various parts, so that after the oil pressure pump has failed, one is mostly compelled to dismount the extruder entirely since presently it is impossible to remove the quantity of plastics, resting in the extruder housing, by further rotation of the mixing and conveying worm(s). Neither can this be done by hand since the sliding faces of the bearing disc on the shaft of the mixing and conveying worm and of the counter bearing ring have been "freezing".
One has tried to reduce damage to bearings in an extruder by using several hydrostatic bearings but nevertheless, the aforementioned difficulties could not be entirely obviated.